Within the integrated circuit industry there is a continuing effort to increase integrated circuit speed as well as device density. As a result of these efforts, there is a trend towards using flip chip technology when packaging complex high speed integrated circuits. Flip chip technology is also known as control collapse chip connection (C4) technology. In C4 technology, the integrated circuit die is flipped upside down. This is opposite to how integrated circuits are packaged today using wire bond technology. By flipping the integrated circuit die upside down, ball bonds may be used to provide direct electrical connections from the bond pads of the die directly to a corresponding set of pads on a package.
In the following discussion reference will be made to a number of drawings. The drawings are provided for descriptive purposes only and are not drawn to scale.
FIG. 1 illustrates an integrated circuit die 102 that is housed in a cavity 105 of a PGA (Pin Grid Array) package 110. The integrated circuit die includes a semiconductor substrate 103 having a top surface 107 and a back side surface 108. The active regions 109 of the integrated circuit are formed from the top surface 107 of the of the semiconductor substrate 103. Wire bonds 104 are used to electrically connect integrated circuit connections in integrated circuit die 102 through internal metal interconnects to the pins 106 of package substrate 110. With the trend towards high speed integrated circuits, the inductance generated in the wire bonds 104 of the typical wire-bonded integrated circuit packaging becomes an increasingly significant problem.
FIG. 2 illustrates a C4 mounted integrated circuit die 202 that is electrically coupled to a PGA (Pin Grid Array) package 210 by ball bonds 204. Die 202 includes a semiconductor substrate 203 that has a top surface 208 and a back side surface 207. The active regions 209 of the integrated circuit are formed from the top surface 208 of the of the semiconductor substrate 203. Because the bond pads of integrated circuit device 202 are located on the top-side surface 208 of the device, the die must be flipped upside down so that it may be attached to package 210. In comparison with the wire bonds 104 of FIG. 1, the ball bonds 204 of integrated circuit device 202 provide more direct electrical connections between the integrated circuit device 202 and the pins 206 of package substrate 210. As a result, the inductance problems associated with typical integrated circuit wire bond packaging technologies are minimized. Unlike wire bond technology, which only allows bonding along the periphery of the integrated circuit die, C4 technology allows connections to be placed anywhere on the integrated circuit die surface. This leads to a much cleaner and more efficient power distribution to the integrated circuit which is another major advantage of C4 technology.
During the silicon debug phase of a new product it is often necessary to probe certain internal portions of the integrated circuit in order to obtain important electrical data from the integrated circuit. Important data include measuring device parameters such as, but are not limited to, voltage levels, timing information, current levels and thermal information. Emissions from the backside of the die may also be measured to determine or locate a variety of defects, such as impact ionization, shorts, hot carrier effects, forward and reverse bias P-N junctions, transistors in saturation and gate oxide breakdown.
Present day debug process for wire bond technology is based on directly probing the metal interconnects on the chip front side with an electron beam (E-beam) or mechanical prober. Typical integrated circuit devices have multiple layers of metal interconnects and it is often difficult to access nodes that are buried deep in the chip. Usually other tools such as plasma etchers and focused ion beam systems must be used to mill away the dielectric and or metal above the node to expose metal nodes for probing. With C4 packaging technology, however, this front side methodology is not feasible since the integrated circuit die is flipped upside down making these internal metal nodes inaccessible.
In order to test and debug C4 mounted integrated circuit devices a number of optical-based testing methods, such as laser probing, have been developed that permit probing of internal portions of an integrated circuit through the back side of the C4 mounted devices. Since the active regions of the integrated circuit are located near the back side surface of the device, it is easier to access these regions through the silicon substrate for the purposes of laser or optical probing and/or for detecting photon emissions emitted from active devices. Since it is often difficult to access the active regions of an integrated circuit from the top surface of a wire-bonded integrated circuit device, it may be desirable to laser probe or detect photon emissions from the back side of a wire-bonded integrated circuit device. As such, the testing of internal nodes may be simplified with the use of an optical-based back side testing methodology. In such an instance, a portion of the package housing the integrated circuit must first be removed to expose the back side surface of the die's semiconductor substrate. Then optical based probing can be performed through the removed portion of the package housing.
During the testing or debugging of an integrated circuit device, it is generally desirable to operate the integrated circuit at its full operating capacity. Since the power density in modern microprocessors is typically very high, it is extremely important to remove heat created by the devices in order for the devices to maintain acceptable operating temperatures. If the temperature of the integrated circuit is not properly controlled, the performance of the circuit may be affected. In some instances, component degradation will occur if the temperature of the integrated circuit is not properly regulated. Thus, any information collected must be obtained with the device operating in its native packaged environment and with its temperature properly regulated. Otherwise, any information obtained may be useless.
FIG. 3A illustrates a prior art approach to dissipating heat from a C4 mounted semiconductor device. Heat is removed from the back side surface 207 of semiconductor device 202 by passing an air flow 240 over a finned heat sink 230 that is thermally coupled to back side surface 207. In some high power applications, heat is dissipated from semiconductor device 202 by attaching a thermally conductive heat slug 232 (e.g., a copper plate) to back side surface 207 and thermally coupling the heat slug to a heat sink. (See FIG. 3B.) In some instances, heat slug 232 is thermally coupled to a metal plate having a large thermal mass and a large heat transfer area. In other instances, heat slug 232 may be thermally coupled to a heat spreading plate by a heat pipe or some other low resistance thermal path.
As depicted in FIG. 4A, the removal of heat from a wire bonded semiconductor device 102 generally involves attaching a finned heat sink 130 to the bottom surface 112 of package 110 and passing an air flow 140 over the heat sink. A heat flow path is established across the back side surface 114 of semiconductor device 102 through package 110 and into heat sink 130. A heat slug (now shown) embedded within package 110 thermally couples die 102 to heat sink 130. Heat is carried away by the air flow 140 passing across finned heat sink 130. In high power applications, a heat slug 130 may be attached to the bottom surface 112 of package 110 and then thermally coupled to a heat spreading plate or other heat sink. (See FIG. 4B.) In the heat removal apparatus of FIGS. 4A and 4B, it is assumed that package 110 is made of a thermally conductive material such as a those typically used in standard ceramic packaging for integrated circuits. Although the prior art heat removal methods described above have proved sufficient in the past, they cannot be used in conjunction with testing methods that require probing through the back side of the semiconductor substrate. In other words, the prior art heat removal methods cannot be used in conjunction with any testing method or process that requires optical access to or through the back side of a semiconductor substrate.
Therefore, what is needed is a method and an apparatus for removing heat from the back side surface of an integrated circuit semiconductor substrate while simultaneously allowing the back side surface of the semiconductor to be probed, or otherwise tested in accordance with an optical-based testing technique.